The effects of energy-level resonance on collision-induced electronic energy transfer: CD (A 2Δ ↔ B 2Σ−) coupling

Abstract

Pulsed, time- and wavelength-resolved laser-induced fluorescence spectroscopy has been used to measure rate constants for collision-induced electronic energy transfer (EET) between the A ²Δ and B ²Σ⁻ states of the CD radical. EET rate constants in the exothermic direction from B ²Σ⁻, v = 0 to the unresolved A ²Δ, v = 0 and 1 levels span the range 0.1–2.4 × 10⁻¹¹ cm³ s⁻¹ at room temperature (ca. 295 K) for the partners He, Ar, N₂, CO and CO₂. H₂ was also investigated, but was unsuitable for further study because of its rapid isotope exchange with CD(X ²Π). As expected, only CO results in a significant rate of removal on any distinct, unobserved channel, presumed to be chemical reaction. The efficient A ²Δ, v = 1 → 0 vibrational relaxation by CO₂ observed previously for CH was not found for CD. Despite the significant differences in their detailed rovibronic level structures, the overall efficiency of EET in CD was found to be very similar to that for CH. The positive correlation in a Parmenter–Seaver plot appears to confirm a role for long-range attractive forces in the EET process. However, the detailed deviations from this overall trend found reproducibly for CD and CH suggests that partner-specific interactions are also important.